The level of biological organization composed of tissues is the organ.

Here's a breakdown of the levels of organization:

1. Cells: The basic unit of life.

2. Tissues: Groups of similar cells working together to perform a specific function (e.g., muscle tissue, nervous tissue).

3. Organs: Structures composed of different tissues that work together to carry out a more complex function (e.g., the heart, the stomach, the brain).

4. Organ systems: Groups of organs that work together to perform major bodily functions (e.g., the digestive system, the circulatory system).

5. Organism: A complete living being made up of multiple organ systems.